Bottom Line:
We studied cation exchange (CE) in core/shell Cu2-xSe/Cu2-xS nanorods with two cations, Ag(+) and Hg(2+), which are known to induce rapid exchange within metal chalcogenide nanocrystals (NCs) at room temperature.At the initial stage of the reaction, the guest ions diffused through the Cu2-xS shell and reached the Cu2-xSe core, replacing first Cu(+) ions within the latter region.For both guest ions, CE is thermodynamically driven as it aims for the formation of the chalcogen phase characterized by the lower solubility under the specific reaction conditions.

ABSTRACTWe studied cation exchange (CE) in core/shell Cu2-xSe/Cu2-xS nanorods with two cations, Ag(+) and Hg(2+), which are known to induce rapid exchange within metal chalcogenide nanocrystals (NCs) at room temperature. At the initial stage of the reaction, the guest ions diffused through the Cu2-xS shell and reached the Cu2-xSe core, replacing first Cu(+) ions within the latter region. These experiments prove that CE in copper chalcogenide NCs is facilitated by the high diffusivity of guest cations in the lattice, such that they can probe the whole host structure and identify the preferred regions where to initiate the exchange. For both guest ions, CE is thermodynamically driven as it aims for the formation of the chalcogen phase characterized by the lower solubility under the specific reaction conditions.

Mentions:
These findings were supported by elemental analyses (viaSTEM-EDS),which yielded a Hg/Se ratio of ∼1 and a Ag/Se ratio of ∼2for the Hg-treated and Ag-treated NRs, respectively (Table S1). High-resolution TEM (HRTEM) images of partiallyexchanged Cu2–xSe/Cu2–xS NRs, with both Ag+ and Hg2+ ions, reported in Figure S5, confirmedthe selective ion replacement in the core region and, importantly,showed a continuous shell, exhibiting no cracks, surrounding the cores.The exposure of Cu2–xSe/Cu2–xS NRs to a higher amount of Ag+ or Hg2+ ions led to an almost complete replacementof Cu+ ions (see Figures S6, S7 and Table S1). The peculiarity of these results stands in the evidencethat cation replacement in the Cu2–xSe core must be preceded, both for Ag+ and Hg2+ cations, by diffusion of cations through the Cu2–xS shell. The structural transformations of the core/shellCu2–xSe/Cu2–xS NRs upon partial CE were monitored via X-ray diffraction(XRD). The XRD patterns of the pristine Cu2–xSe/Cu2–xS NRs (Figure 3c) were dominatedby the hexagonal Cu2S (high chalcocite) peaks, with otherminor reflections ascribable to the metastable Cu2Se “chalcocite-like”phase, as previously reported by us.7a TheXRD patterns of partially exchanged NRs, instead, evidenced in bothcases the presence of high chalcocite Cu2S, together witha second majority phase that, in the case of Ag+-treatedNRs, could be indexed according to the orthorhombic Ag2Se (naummanite) phase (see Figure 3b), while for the Hg2+-treated rods, itcould not be indexed to any known bulk HgSe or to any alloyed HgSexS1–x phase.

Mentions:
These findings were supported by elemental analyses (viaSTEM-EDS),which yielded a Hg/Se ratio of ∼1 and a Ag/Se ratio of ∼2for the Hg-treated and Ag-treated NRs, respectively (Table S1). High-resolution TEM (HRTEM) images of partiallyexchanged Cu2–xSe/Cu2–xS NRs, with both Ag+ and Hg2+ ions, reported in Figure S5, confirmedthe selective ion replacement in the core region and, importantly,showed a continuous shell, exhibiting no cracks, surrounding the cores.The exposure of Cu2–xSe/Cu2–xS NRs to a higher amount of Ag+ or Hg2+ ions led to an almost complete replacementof Cu+ ions (see Figures S6, S7 and Table S1). The peculiarity of these results stands in the evidencethat cation replacement in the Cu2–xSe core must be preceded, both for Ag+ and Hg2+ cations, by diffusion of cations through the Cu2–xS shell. The structural transformations of the core/shellCu2–xSe/Cu2–xS NRs upon partial CE were monitored via X-ray diffraction(XRD). The XRD patterns of the pristine Cu2–xSe/Cu2–xS NRs (Figure 3c) were dominatedby the hexagonal Cu2S (high chalcocite) peaks, with otherminor reflections ascribable to the metastable Cu2Se “chalcocite-like”phase, as previously reported by us.7a TheXRD patterns of partially exchanged NRs, instead, evidenced in bothcases the presence of high chalcocite Cu2S, together witha second majority phase that, in the case of Ag+-treatedNRs, could be indexed according to the orthorhombic Ag2Se (naummanite) phase (see Figure 3b), while for the Hg2+-treated rods, itcould not be indexed to any known bulk HgSe or to any alloyed HgSexS1–x phase.

Bottom Line:
We studied cation exchange (CE) in core/shell Cu2-xSe/Cu2-xS nanorods with two cations, Ag(+) and Hg(2+), which are known to induce rapid exchange within metal chalcogenide nanocrystals (NCs) at room temperature.At the initial stage of the reaction, the guest ions diffused through the Cu2-xS shell and reached the Cu2-xSe core, replacing first Cu(+) ions within the latter region.For both guest ions, CE is thermodynamically driven as it aims for the formation of the chalcogen phase characterized by the lower solubility under the specific reaction conditions.

ABSTRACTWe studied cation exchange (CE) in core/shell Cu2-xSe/Cu2-xS nanorods with two cations, Ag(+) and Hg(2+), which are known to induce rapid exchange within metal chalcogenide nanocrystals (NCs) at room temperature. At the initial stage of the reaction, the guest ions diffused through the Cu2-xS shell and reached the Cu2-xSe core, replacing first Cu(+) ions within the latter region. These experiments prove that CE in copper chalcogenide NCs is facilitated by the high diffusivity of guest cations in the lattice, such that they can probe the whole host structure and identify the preferred regions where to initiate the exchange. For both guest ions, CE is thermodynamically driven as it aims for the formation of the chalcogen phase characterized by the lower solubility under the specific reaction conditions.